Save as or Export IDML. which can be opened by versions CS4 and later, if they are fully updated.
I tried, but after I saved it to IDML, the file become smaller and the other party who use CS5 can not open it
Smaller is normal. IDML strips out a lot of information, including image previews. To open in CS5 they must have the 7.0.4 for InDesign installed.
And don't expect a really accurate conversion. If you used any features in CC 2014 that are not available in CS5 they will be lost, and there is likely to be some change in the text flow.